The inkjet recording system is a printing system in which a liquid ink having high fluidity is ejected from a fine nozzle and attached on a recording medium, for example, paper, to perform printing. The system has a feature in that an image having high resolution and high quality can be printed at a high speed and with a low noise using a relatively inexpensive device and has rapidly become widespread. Since a colorant of the ink used in the inkjet recording system is superior in light fastness, weather fastness and water resistance required for high image quality printing, inks containing pigment as the colorant are increasing.
Color printing in inkjet recording is ordinarily performed by expressing various hues by using a three color ink set composed of pigment inks of the three colors yellow (Y), magenta (M) and cyan (C) which are the three primary colors in subtractive color mixing, or a four color ink set that includes black (K) in addition to these three colors. Since pigments may differ in the structure, surface treatment or particle size thereof; when two kinds of pigments are used in one ink (for example, Y ink), dispersion states of the pigments change, leading to deterioration of storage stability, and causing changes in tint. Therefore, a single pigment is ordinarily used in the respective inks of Y, M, C and K.
In aqueous inks, on the other hand, it has been attempted to use plural kinds of pigments in one ink from the standpoints of color-forming property and glossiness. For example, in Patent Document 1, it is described that the color-forming property of an aqueous ink is improved by using a mixture of pigment dispersions dispersed by different dispersion means. Also, in Patent Document 2, it is described that the glossiness in the case of printing on glossy paper is improved by microencapsulation of a pigment mixture composed of plural kinds of organic pigments using a resin having a dispersing function.